The RNG
Nowadays slot machines are computerized. The odds are programmed. In modern slot machines, the reels and lever are present for historical and entertainment reasons only. The positions of the reels are chosen by a Random Number Generator. It is included into the machine's software.
The RNG is constantly generating random numbers at an extremely high rate. When you pres the Play button the result is determined by the most recent random number. The result varies depending on exactly when the game is played. A fraction of a second earlier or later, and the result would be different.
The RNG was noticed to generate not exactly random numbers. It turned out that The number sequence is repeated time after time. Poor programming determines such behavior. Pseudo RNGs with very long periods are relatively easy to build. There is no computer that is able to complete a single period in the expected lifetime of the universe. It was Ronald Dale Harris, a former slot machine programmer, who knew the pseudo RNG code and seed values. Equations for specific gambling games like Keno were discovered by him. They allowed to predict that the next set of selected numbers would be based on the previous games played. But the RNG picks numbers even when the machine is not being played. So the player cannot see the secquence.

Payout Percentage
The percentage of winnings paid out of the money that is wagered by players is 82–98 per cent. The term theoretical payout percentage is used to denote it. The minimum theoretical payout percentage varies among jurisdictions. Its establishment refers to the sphere of law or regulation. A certain winning pattern exists nearly in every casino. It contains the information about the amounts they pay and the frequencies of these pay-outs. The winning patterns on slot machines are carefully selected to yield a certain fraction of the money played to the house. As for the rest of the money it goes to the players.
A slot machine's software is written and the theoretical payout percentage is set simultaneously. Changing the payout percentage after a slot machine has been placed on the gaming floor requires a physical swap of the software. Being a time-consuming process it is done not very often. In certain jurisdictions the EPROM has a tamper-evident seal. Gaming Control Board officials should be called for changing it. Other jurisdictions randomly audit slot machines to ensure that they contain only approved software.
Thanks to the newly developed technology the game, the odds, and the payouts would be able to be changed by the casino's slot manager remotely. The change cannot be done instantaneously, but only after the selected machine has been idle for at least four minutes. After the change is made, the machine must be locked to new players for four minutes and display an on-screen message informing potential players that a change is being made.

Machines That Are Linked
A group of machines offers a particularly large prize, or jackpot if they are linked together in a special way. A small amount is contributed by each machine in the group to this progressive jackpot which is given to a player who gets a specific combination of symbols. The amount paid for the progressive jackpot is usually far higher than any single slot machine could pay on its own.
Sometimes multiple machines form multiple casinos. In these cases the manufacturer owning these machines is responsible for paying the jackpot.

Near-miss Programming
The reel display of modern slot machines is controlled by computer software. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
This practice of showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
This term is also used for a related phenomenon. The chance of a winning combination appearing on a pay line is controlled by the winning percentages programmed into the slot machine. The combinations above and below the payline are all roughly equally randomly distributed. This means it is much more likely that a winning combination will appear above or below a pay line than on the pay line. This phenomenon only occurs where abbreviated physical reels are used to display a win pattern based upon the RNG. In video slot machines, virtual reels are used and the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
The issue of a near-miss above or below the pay line was also the subject of the Nevada Gaming Commission investigation. They ruled that this was legal, so long as the near-miss above or below the pay line was not specially programmed. The near-miss and any other combination should have the same possibility of occurrence. The machine can't show winning combinations more frequently than other combinations above or below the pay line.
In Australia the usage of near-miss programming, where a near miss is inaccurately displayed is also illegal. To audit manufacturer's practices regulators use stop motion cameras.

Fraud
Mechanical slot machines and their coin acceptors were sometimes considered to be cheating devices and other scams.
EPROM computer chips controls modern slot machines and coin acceptors have become obsolete in favor of bill acceptors. These machines and their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. Recent attempts at defrauding slot machines involve manipulating the EPROM, such as by directing microwaves toward it to disrupt its proper functioning.